So I can buy either green or blue flex cable keyboard? Thank you?

This model appears to take only one keyboard - so it should not matter.

Thank you so much.

You may find that you will have issues when you start using the wrong keyboard. I believe the blue cable was for the M17x R3 / M18x R1 and the green was for the M17x R4 / M18x R2.

While things may seen to work ok at the start, other people have found that problems start to happen soon after as they are not 100% compatible. You really need to get the correct one for your model. 

I just remember reading a thread with someone saying they had issues using an R3 keyboard on an R4. From memory, some of the keys started to stop working. Check the Part#'s at Parts-People just to be sure. You may not have any issue at all but there's a chance.

There are also eBay sellers that say that they can't be interchanged. Like this one

I've got R3's and R4's but I've never swapped the keyboards around to test it myself. I just sold my last R4 today so I still won't be able to test. 

Oh thank you. But the keyboard that attached on mine is blue cable.  

On the Parts-People.com, The r4 keyboard cable is color blue on their disassembly project. Can you tell me what's your r4 color of your cable keyboard?

I've wrapped up my R4 ready to send off but I have some R3's at home so I'll check their cables. Oh, and I have a few M18x R2's which have the same keyboard as the M17x R4 so I'll check those after work tonight. 

I'll let you know ASAP but it won't be for 4-5 hours. 

Oh thanks for the disturbance. I'll keep touch on you. Keep safe!